1. Field of the Invention
This invention relates to a multi-input power supply circuit and a method of switching power supplies of the multi-input power supply circuit and particularly, to a multi-input power supply circuit including one transformer that has windings corresponding to a plurality of power supplies and a method of switching the power supplies of the multi-input power supply circuit.
2. Description of the Related Art
There is a multi-input power supply circuit provided with a plurality of power supply inputs, which include both an AC 100 V alternating-current input and a 12 V direct-current input, for example. For instance a power supply circuit including one transformer provided with windings corresponding to a plurality of power supplies is described in Japan Utility Model Publication No 07-009089 (FIG. 1 and Paragraph [0012]), JP-A-2002-233079 (Paragraph [0011] and FIG. 3), and JP-A-11-089113 (FIG. 6 and Paragraphs [0003] to [0005]). Further, JP-A-2002-233679 describes a technique of overlapping switching operations when switching is made among a plurality of power supply inputs.
In this type of multi-input power supply circuit, input power supplies are required to have high dielectric strength in some cases, depending on types of the input power supplies.
The conventional multi-input power supply circuit characterized by having one transformer has a disadvantage in that insulation cannot be performed when an attempt is made to supply electric power to a load circuit without a momentary interruption. Meanwhile, if a plurality of insulated windings and switching circuits are used to secure insulation, a delay time is generated when switching is made among switching systems, and thus prevention of the momentary interruption cannot be achieved.
Further, if the delay time caused by the switching is reduced too much, there arises a time period in which switching devices of two or more switching systems are turned ON at the same time. This causes a disadvantage in that the electric power returns to an input system of another system, the electric power is not normally supplied to the load circuit, and thus the momentary interruption of the electric power supply is caused.
Furthermore, another type of circuit including a combination of a plurality of converters has problems in that the size of the circuit is increased and in that efficiency of the circuit is decreased.